In general, a hydraulic excavator which is a typical example of a construction machine is composed of an automotive lower traveling structure and an upper revolving structure rotatably mounted on the lower traveling structure. A working mechanism performing excavating work of earth and sand and the like is tiltably provided on the front side of a revolving frame constituting the upper revolving structure.
Here, the working mechanism of a hydraulic excavator is composed mainly of a boom having the base end side rotatably mounted on the revolving frame, an arm rotatably mounted on the distal end side of the boom, a working tool such as a bucket or the like rotatably mounted on the distal end side of the arm, and a boom cylinder, an arm cylinder, and a bucket cylinder driving the boom, the arm, and the bucket, respectively.
The arm constituting the working mechanism has a closed sectional structure having a square cross section and is formed as a lengthy box-shaped structural body whose whole length is as long as several meters. That is, the arm is formed of left and right side plates, an upper plate joined to the upper end sides of these left and right side plates by welding, a lower plate joined to the lower end sides of the left and right side plates by welding, and a rear plate joined to the rear end sides of the left and right side plates and the upper plate by welding as a box-shaped structural body having a closed sectional structure having a square cross section.
Here, a boom connecting boss is joined to the rear sides of the left and right side plates constituting the arm by welding, and a connecting pin rotatably connecting the boom and the arm to each other is inserted into the boom connecting boss. Moreover, an arm cylinder bracket is joined to the rear plate constituting the arm by welding, and a distal end side of the arm cylinder having a base end side mounted on the boom is connected to this arm cylinder bracket through the connecting pin.
As the left and right side plates, the upper plate, the lower plate, and the rear plate constituting the arm, a soft steel material, for example, a rolled steel material for a general structure such as SS400 and the like is usually used. By welding the left and right side plates, the upper plate, the lower plate, and the rear plate made of this soft steel material to each other, an arm made of a firm box-shaped structural body can be formed.
A boom in which the left and right side plates constituting the boom of a hydraulic excavator or the like are formed by joining three members (plate materials), that is, a first member, a second member, and a third member is proposed. In this boom, the second member located in a middle part in a length direction and easily subjected to buckling is formed by using a material with yield stress higher than those of the first and third members. As a result, a plate thickness of the second member can be made thinner than the first, second, and third members formed by using the same material, and weight reduction of the boom can be realized (Patent Document 1).
Therefore, when a high tensile steel material with larger tensile strength than the soft steel material is used as the left and right side plates, the upper plate, the lower plate, and the rear plate constituting the arm, a plate thickness of the left and right side plates, the upper plate, the lower plate, and the rear plate made of the high tensile steel material can be made smaller than the plate thickness of the left and right side plates, the upper plate, the lower plate, and the rear plate made of the soft steel material. Thus, the arm formed by using the steel plate made of the high tensile steel material can realize weight reduction while maintaining strength equal to the arm formed by using the steel plate made of the soft steel material.